The enzyme required for carboxylation in the Calvin cycle is

Question

Accepted Answer

C. 1. **Recall the first step of the Calvin cycle**, since carboxylation is the entry point where $CO_2$ gets attached to an acceptor molecule.

Given that the acceptor here is RuBP (Ribulose bisphosphate), the reaction is:
$$
RuBP + CO_2 ightarrow 2 	imes 	ext{3-PGA}
$$

2. **Identify the enzyme catalyzing this step**, since every biochemical reaction needs a specific enzyme.

This carboxylation (and also the oxygenation in photorespiration) is catalyzed by **RuBP carboxylase-oxygenase (RuBisCO)** — the same enzyme handles both functions depending on $CO_2$ or $O_2$ availability.

3. **Rule out the other options.**

Hexokinase acts in glycolysis, PEP carboxylase is used in $C_4$ carboxylation (not the Calvin cycle directly), and carboxypeptidase is a protein-digesting enzyme, so none of these fit.

Hence, the enzyme required for carboxylation in the Calvin cycle is **RuBP carboxylase-oxygenase**, so the correct option is **C**.